Diamondback moth (DBM), Plutella xylostella (Linnaeus), is a notorious pest of brassica crops worldwide and is resistant to all groups of insecticides. The insect system harbors diverse groups of microbiota, which in turn helps in enzymatic degradation of xenobiotic-like insecticides. The present study aimed to determine the diversity of gut microflora in DBM, quantify esterase activity and elucidate their possible role in degradation of indoxacarb. We screened 11 geographic populations of DBM in India and analyzed them for bacterial diversity. The culturable gut bacterial flora underwent molecular characterization with 16S rRNA. We obtained 25 bacterial isolates from larvae (n = 13) and adults (n = 12) of DBM. In larval gut isolates, gammaproteobacteria was the most abundant (76%), followed by bacilli (15.4%). Molecular characterization placed adult gut bacterial strains into three major classes based on abundance: gammaproteobacteria (66%), bacilli (16.7%) and flavobacteria (16.7%). Esterase activity from 19 gut bacterial isolates ranged from 0.072 to 2.32 μmol/min/mg protein. Esterase bands were observed in 15 bacterial strains and the banding pattern differed in Bacillus cereus – KC985225 and Pantoea agglomerans – KC985229. The bands were characterized as carboxylesterase with profenofos used as an inhibitor. Minimal media study showed that B. cereus degraded indoxacarb up to 20%, so it could use indoxacarb for metabolism and growth. Furthermore, esterase activity was greater with minimal media than control media: 1.87 versus 0.26 μmol/min/mg protein. Apart from the insect esterases, bacterial carboxylesterase may aid in the degradation of insecticides in DBM.

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ABSTRACT:The toxic effect of commonly used insecticides in cotton fields was studied on 9 populations of Chrysoperla zastrowi sillemi (Esben-Petersen), an important predator of sucking pests collected in India. The dose mortality bioassay against 3-days old larvae was determined using three insecticides viz., endosulfan, fenvalerate and acephate by topical bioassay method. Mechanism of resistance to the above mentioned insecticides were determined without and with three metabolic inhibitors (synergists), viz., piperonyl butoxide (PBO), S,S,S-tributyl-phosphorotrithioate (DEF) and diethyl maleate (DEM). Among the populations, resistant ratios (RR) of CZS-8 was significantly higher i.e. 50.36., 66.11 and 277.51-fold for endosulfan, fenvalerate and acephate, respectively compared to susceptible population (CZS-10). The CZS-8 was selected for synergism study it showed higher LC 50 values and resistance ratio for all three insecticides. It showed 8.97-fold, 18.49-fold and 6.38-fold increase in synergism ratio for endosulfan indicating the resistance was strongly synergised by PBO, DEF and DEM. Similarly for fenvalerate, CZS-8 showed 8.69-fold and 3.63-fold significant increase in synergism ratio by DEF and DEM, respectively and for acephate, CZS-8 showed 54.82-fold, 150.87-fold and 113.52-fold significant increase in synergism ratio indicating that the resistance could be due to cytochrome p-450, esterase and glutathione s-transferase activity. The study indicated that the field population of C. z. sillemi developed resistance to different groups of insecticides. Among different geographical populations, CZS-8 collected from Sriganganagar, was recorded as most resistant.

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